Method for pre-treatment of gold-bearing oxide ores

ABSTRACT

The disclosure relates to pre-treatment of precious metal-bearing oxide ores, prior to precious metal leaching by thiosulfate. The process comprises mixing oxide ore in oxygenated water in the presence of a carbon-based material (e.g., activated carbon or other type of carbon). The carbon-based material can be separated from the ore slurry, and, the gold is thereafter leached by a thiosulfate lixiviant.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S.application Ser. No. 15/729,961 filed Oct. 11, 2017, now issued as U.S.Pat. No. 10,597,752, which is a continuation of U.S. application Ser.No. 14/287,889 with a filing date of May 27, 2014, now issued as U.S.Pat. No. 10,161,016, which claims the benefits of U.S. ProvisionalApplication Ser. No. 61/828,558, filed May 29, 2013, each entitled“Method for Pre-Treatment of Gold-Bearing Oxide Ores”, each of which isincorporated herein by this reference in its entirety.

FIELD

The disclosure relates generally to precious metal recovery fromprecious metal-containing materials and particularly to gold recoveryfrom gold-containing materials.

BACKGROUND

The conventional cyanidation/carbon in pulp process has been the maingold extraction method for decades. While cyanidation is effective forleaching gold from some carbonaceous or complex ores, there are seriousenvironmental concerns associated with the use of cyanide in goldleaching processes. Thiosulfate is among the more successful alternativelixiviants for effective leaching of gold. An example of a thiosulfateleaching process for precious metal-containing materials is shown inU.S. Pat. No. 7,544,232.

Some oxide ores may be refractory in nature. They neither yieldsufficient gold leaching in a thiosulfate leach system nor are leachedas effectively compared to cyanide. Thiosulfate gold extraction fromsome oxide ores can be minimal. As oxide ores do not contain sulfides(or have very low levels of sulfide), the refractory nature cannot bemitigated in the same manner as for sulfide ores (e.g., by roasting,bio-oxidation or pressure oxidation).

There is a need for a thiosulfate leaching method to address therefractory nature of certain oxide ores in the thiosulfate leach system.

SUMMARY

These and other needs are addressed by the various aspects, embodiments,and configurations of the present disclosure. The disclosure is directedgenerally to pre-treatment of precious metal-containing materials priorto thiosulfate precious metal leaching.

A pre-treatment process can include the steps of:

(a) contacting a precious metal-containing material with carbon and anoxidant (e.g., a molecular oxygen-containing gas) to form a pre-treatedslurry;

(b) optionally removing the carbon from the pre-treated slurry to form acarbon-depleted slurry (e.g., having substantially less carbon than thepre-treated slurry); and

(c) contacting the pre-treated slurry or carbon-depleted slurry withthiosulfate to leach a precious metal from the pre-treated preciousmetal-containing material.

The precious metal, for example, can be gold.

Whether or not carbon is removed depends on the particle size of thecarbon employed. When coarse carbon is employed, the carbon is typicallyremoved before thiosulfate leaching. When fine carbon is employed, thecarbon is typically not removed before thiosulfate leaching.

Finely sized carbon can be contacted with the precious metal-containingmaterial either separately after grinding of the material or beforeand/or during grinding. In the latter case, the carbon particles can becoarsely sized but are ground to a fine size distribution similar to asize distribution of the ground precious metal-containing material.

Prior to leaching in step (c), the precious metal-containing materialcan be substantially free of contact with thiosulfate. Stateddifferently, the slurried precious metal-containing material, before andduring step (a), typically includes less than 0.005, more typically nomore than about 0.0025, and even more typically no more than about 0.001molar thiosulfate. In some applications, no thiosulfate or otherlixiviant is contacted with the precious metal-containing materialbefore or during pre-treatment in step (a).

The precious metal-containing material can be amenable to cyanideleaching (and therefore is not cyanide refractory) but not tothiosulfate leaching (i.e., the material is a thiosulfate refractoryprecious metal-containing material). In other words, leaching ofprecious metals from the precious metal-containing material by cyanidecan be more effective than precious metal leaching by thiosulfate. Evenwhen leaching of the precious metal-containing material has similarprecious metal recoveries using either cyanide or thiosulfate as thelixiviant, the pretreatment process can enhance further precious metalrecovery by thiosulfate. The precious metal-containing material may ormay not be concentrated. Generally, the precious metal is in a matrixthat is predominantly one or more oxides. By way of example, theprecious metal-containing material can contain more oxides thansulfides.

The slurry before pretreatment and the pre-treated slurry can each havea pH about pH 3 or higher (and, in some cases, about pH 7 or higher); anoxidation-reduction potential during pretreatment ranging from about 100to about 600 mV (Ag/AgCl electrode); and/or a rate of contact of amolecular oxygen-containing gas with the slurry during pretreatment ofabout 0.10 L O₂/L slurry/min or higher.

Generally, a weight ratio of the precious metal-bearing material tocarbon ranges from about 50:1 to about 1:0.01 but the amount of carbonemployed in any application can depend on the carbon particle size. Aweight ratio of the precious metal-bearing material to coarsely sizedcarbon commonly ranges from about 1:5 to about 1:0.01 and more commonlyfrom about 1:3 to about 1:0.5. A weight ratio of the preciousmetal-bearing material to finely sized carbon commonly ranges from about1:1 to about 50:1 and more commonly from about 10:1 to about 30:1.

The pre-treatment process can be carried out under ambient conditions(room temperature and atmospheric pressure) in less than 24 hours.Increasing the process temperature can further improve the gold recoveryand/or pretreatment kinetics.

The carbon is normally removed from the pre-treated slurry by screening,which generally requires about 95% or more, and even more commonly about98% or more of the carbon to be retained on the screen while about 90%or more and more commonly about 95% or more of the preciousmetal-containing material passes through the screen. The relative mean,median, mode, and P₈₀ particle sizes of the carbon and preciousmetal-containing material are selected to produce at least these levelsof separation.

After carbon separation, the discharge slurry from the pre-treatmentprocess, can be directly advanced to thiosulfate leaching. Thecarbon-depleted slurry can be contacted with thiosulfate in thesubstantial absence of pH adjustment and/or slurry density adjustment.As an example, a pH of the carbon-depleted slurry is commonly adjustedby no more than about pH 0.1 and the slurry density by no more thanabout 5%.

The present disclosure can provide a number of advantages depending onthe particular configuration. Pre-treating oxide ores in oxygenatedwater in the presence of activated carbon or other carbon-basedmaterials can improve significantly the gold recovery by thiosulfateleaching. The process can have a low operating cost and provide astraightforward pre-treatment method for oxide ores to be followed bythiosulfate leaching of gold. Attrition, due to mixing of the slurry, iscommonly the only cause for carbon loss and may be minimized by properengineering of the agitators and reactors. The carbon-based material canbe recycled and re-used, thereby decreasing operating costs. Inexpensiveair (or more expensive oxygen gas) are the only reagents consumed,thereby making the economics of the process very attractive.

These and other advantages will be apparent from the disclosure of theaspects, embodiments, and configurations contained herein.

As used herein, “at least one”, “one or more”, and “and/or” areopen-ended expressions that are both conjunctive and disjunctive inoperation. For example, each of the expressions “at least one of A, Band C”, “at least one of A, B, or C”, “one or more of A, B, and C”, “oneor more of A, B, or C” and “A, B, and/or C” means A alone, B alone, Calone, A and B together, A and C together, B and C together, or A, B andC together. When each one of A, B, and C in the above expressions refersto an element, such as X, Y, and Z, or class of elements, such asX₁-X_(n), Y₁-Y_(m), and Z₁-Z_(o), the phrase is intended to refer to asingle element selected from X, Y, and Z, a combination of elementsselected from the same class (e.g., X₁ and X₂) as well as a combinationof elements selected from two or more classes (e.g., Y₁ and Z_(o)).

The term “a” or “an” entity refers to one or more of that entity. Assuch, the terms “a” (or “an”), “one or more” and “at least one” can beused interchangeably herein. It is also to be noted that the terms“comprising”, “including”, and “having” can be used interchangeably.

The term “activated carbon” is a form of carbon processed to be riddledwith small, low-volume pores that increase the surface area availablefor adsorption or chemical reactions. Activated carbon can be granular,extruded, bead, impregnated, and/or polymer coated.

The term “carbon” includes a carbon-containing organic material, such asone or more of activated carbon (or activated charcoal or activatedcoal), coal (e.g., peat, lignite, sub-bituminous coal, bituminous coal,steam coal, anthracite, and graphite), brown coal, coke, hard carbonderived from coconut shells or elemental carbon, a calcined resin, andmixtures thereof.

The term “means” as used herein shall be given its broadest possibleinterpretation in accordance with 35 U.S.C., Section 112, Paragraph 6.Accordingly, a claim incorporating the term “means” shall cover allstructures, materials, or acts set forth herein, and all of theequivalents thereof. Further, the structures, materials or acts and theequivalents thereof shall include all those described in the summary ofthe invention, brief description of the drawings, detailed description,abstract, and claims themselves.

The term “precious metal” refers to gold and silver.

A “thiosulfate refractory” precious metal-containing material is amaterial in which at least part of the precious metal-containingmaterial is naturally resistant to recovery by thiosulfate leaching. Therecovery of thiosulfate refractory ores can be increased by pretreatmentprior to thiosulfate leaching, or by employing cyanide leaching.

Unless otherwise noted, all component or composition levels are inreference to the active portion of that component or composition and areexclusive of impurities, for example, residual solvents or by-products,which may be present in commercially available sources of suchcomponents or compositions.

All percentages and ratios are calculated by total composition weight,unless indicated otherwise.

It should be understood that every maximum numerical limitation giventhroughout this disclosure is deemed to include each and every lowernumerical limitation as an alternative, as if such lower numericallimitations were expressly written herein. Every minimum numericallimitation given throughout this disclosure is deemed to include eachand every higher numerical limitation as an alternative, as if suchhigher numerical limitations were expressly written herein. Everynumerical range given throughout this disclosure is deemed to includeeach and every narrower numerical range that falls within such broadernumerical range, as if such narrower numerical ranges were all expresslywritten herein. By way of example, the phrase from about 2 to about 4includes the whole number and/or integer ranges from about 2 to about 3,from about 3 to about 4 and each possible range based on real (e.g.,irrational and/or rational) numbers, such as from about 2.1 to about4.9, from about 2.1 to about 3.4, and so on.

The preceding is a simplified summary of the disclosure to provide anunderstanding of some aspects of the disclosure. This summary is neitheran extensive nor exhaustive overview of the disclosure and its variousaspects, embodiments, and configurations. It is intended neither toidentify key or critical elements of the disclosure nor to delineate thescope of the disclosure but to present selected concepts of thedisclosure in a simplified form as an introduction to the more detaileddescription presented below. As will be appreciated, other aspects,embodiments, and configurations of the disclosure are possibleutilizing, alone or in combination, one or more of the features setforth above or described in detail below. Also, while the disclosure ispresented in terms of exemplary embodiments, it should be appreciatedthat individual aspects of the disclosure can be separately claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawing is incorporated into and forms a part of thespecification to illustrate several examples of the present disclosure.This drawing, together with the description, explains the principles ofthe disclosure. The drawing simply illustrates preferred and alternativeexamples of how the disclosure can be made and used and is not to beconstrued as limiting the disclosure to only the illustrated anddescribed examples. Further features and advantages will become apparentfrom the following, more detailed, description of the various aspects,embodiments, and configurations of the disclosure, as illustrated by thedrawings referenced below.

FIG. 1 is a process flow schematic according to an embodiment of thedisclosure.

DETAILED DESCRIPTION Overview

The present disclosure provides a process for pre-treating preciousmetal-bearing materials. The process can be performed prior tothiosulfate leaching and improve the overall precious metal recovery ofthiosulfate refractory precious metal-containing materials. Thepre-treatment is done by mixing a slurry containing the preciousmetal-containing material, water, a carbon-based material, and dissolvedmolecular oxygen (as the oxidizing reagent) for a predeterminedresidence time.

The precious metal-bearing material can be an oxide ore, concentrate,tailings, leach residue, calcine, and other precious metal-bearing oxidematerials. Typical precious metal-bearing oxide ores and concentratesmay contain silicates, phosphates, iron oxides, and hydroxides, andrelatively low levels of residual sulfides.

In the pre-treatment process, the precious metal-bearing material ismixed, in a stirred tank, vat, or other suitable reactor, with thecarbon-based material, such as activated carbon, and water to form theslurry. Molecular oxygen is typically contacted by sparging the slurry.The molecular oxygen can be supplied by a suitable source, such as air,oxygen-enriched air, or industrially-pure oxygen, with ambient air beingpreferred. The process can be carried out in any water source, whetherraw water or relatively clean process water. Other suitable reactors,such as pulse columns, can be any reactor able to adequately mix carbon,the slurried precious metal-containing material, and gas.

Proper reaction conditions can provide relatively high kinetics.Typically, the pre-treatment process is conducted at atmosphericpressure and temperature, though the use of a higher operatingtemperature (e.g., typically about 35° C. or higher and more typicallyabout 50° C. or higher) can provide improved reaction kinetics. The pHof the slurry is typically about pH 7 or higher, more typically about pH8 or higher, and even more typically about pH 9 or higher. Theoxidation-reduction potential (“ORP”) of the slurry is typically greaterthan about 100 mV and more typically greater than about 200 mV andtypically less than about 750 mV and more typically less than about 500mV (Ag/AgCl electrode). The rate of sparging of molecular oxygen throughthe slurry during pre-treatment typically ranges from about 0.05 toabout 5 and more typically from about 0.10 to about 2.5 L O₂/Lslurry/min. The residence time of the slurry in the mixing vesseltypically ranges from more than about 1 hour to about 24 hours,depending on the temperature, dissolved oxygen concentration insolution, and the ore type. Ultimately, the pre-treatment conditions,particularly time and temperature of the pretreatment process,carbon-based material dosage, and rate of oxygen addition, are adjustedto optimize precious metal recovery.

The weight ratio of the carbon-based material to the preciousmetal-bearing material can vary depending on the requirements of thespecific ore, the properties of the carbon-based material itself, andthe desired level of precious metal recovery. Typically, for coarselysized carbon the weight ratio of the precious metal-bearing material tothe carbon-based material ranges from about 1:3 to about 1:0.01 and moretypically from about 1:3 to about 1:0.1. A more typical weight ratio ofthe precious metal-containing material to the coarsely sizedcarbon-based material is about 1:0.5. Typically, for finely sized carbonthe weight ratio of the precious metal-bearing material to thecarbon-based material commonly ranges from about 1:1 to about 50:1 andmore commonly from about 10:1 to about 30:1. A more typical weight ratioof the precious metal-containing material to the finely sizedcarbon-based material is about 20:1. The carbon-based material isgenerally not consumed in the pre-treatment process and can be recycledand re-used, with make-up for carbon attrition. Oxygen gas is commonlythe only reagent consumed though any other oxidant, including ozone anda peroxygen compound such as hydrogen peroxide, may be employed.

The process can be carried out batch-wise or continuously, the latterbeing preferred.

After pre-treatment is completed, the carbon-based material can beseparated by a suitable technique from the pre-treated preciousmetal-bearing material in the pre-treated slurry. Separation isgenerally done in applications using coarsely sized carbon particles butnot finely sized carbon particles. Coarsely sized carbon particleseparation may be done using differences in particle size. To make thiseffective, a considerable particle size difference between the coarselysized particles of the carbon-based material and the more finely sizedparticles of the precious metal-containing material is normallyrequired. Regardless of the separation technique employed, the coarselysized carbon-based material may be recycled many times to thepre-treatment process.

Unlike operations using coarsely sized carbon particles, operationsusing finely sized carbon particles generally do not separate the carbonparticles from the particles of the pre-treated preciousmetal-containing material. After precious metal recovery, the finelysized carbon particles are sent to tailings along with the preciousmetal barren material.

The pre-treated slurry can then be fed directly to the thiosulfateleaching process. No filtration of the slurry before thiosulfateleaching is generally required. Depending on the ore type, thepre-treated slurry commonly has pH greater than about pH 3, morecommonly greater than about pH 7, and even more commonly greater thanabout pH 8. In some cases, no pH adjustment is required before thepre-treated slurry is contacted with the thiosulfate lixiviant tocommence leaching. As will be appreciated, thiosulfate leaching isgenerally performed at a pH of between about pH 7.5 and pH 10.

The method of the present invention is particularly suitable forpre-treatment of gold-bearing oxide ores and concentrates, prior tothiosulfate leaching, to improve the gold recovery of the thiosulfateleaching process. Direct thiosulfate leaching of some gold-bearing orescan result in poor gold recovery, and pre-treatment before the leachingprocess can provide a substantial increase in gold recovery.

Exemplary Precious Metal Pre-Treatment and Recovery Process

FIG. 1 is an exemplary schematic flow diagram depicting the unitoperations of gold-bearing oxide ore pre-treatment prior to thiosulfateleaching. The process generally pre-treats the gold-bearing ore with acarbon-based material (e.g., activated carbon) in oxygenated-water,optionally removes the carbon-based material after pre-treatment, andfeeds the pre-treated slurry directly to the thiosulfate leachingprocess. While discussed with reference to gold-bearing oxide ores, theprocess can be applied to any type of precious metal-bearing material.

Referring to FIG. 1, the precious metal-bearing material 100 is mixed,in step 116, with water 104 in the mixer unit (not shown) to form aslurry to be pre-treated. Although no pH adjustment is generallyrequired, the need for pH adjustment depends on the material'scomposition and the ratio of the material, water and carbon in theslurry. The pH can increase during pretreatment. The initial pH can beacidic or basic, depending on the application. For example, the initialpH commonly ranges from about pH 3 to about pH 9. An increase in pHtypically to a final pH of about pH 7 to about pH 10 and more typicallyof about pH 7 to about pH 9, has been observed, which can allow thethiosulfate leaching to proceed without any (or in the absence of) pHadjustment prior to contact of the thiosulfate lixiviant 132 with thepre-treated precious metal-bearing material.

In step 116, fresh and/or recycled carbon-based material 128 and anoxidant 198 (e.g., molecular oxygen/air or enriched air) are contactedwith the slurry in the mixer unit. The mixer commonly mixes the variousslurry constituents at the ambient temperature and atmospheric pressurein an oxygenated condition. The oxidant 108 can be supplied by the useof air, oxygen-enriched air, or pure oxygen and the non-reacted portionof the oxidant gas may be vented as off-gas 112. The residence time ofthe slurry in the mixer unit depends on the material type and can rangefrom about 1 hr to about 24 hrs.

In one configuration, the carbon-based material is comminuted with theprecious metal-bearing material before pretreatment. In that event, asize distribution of the comminuted precious metal-bearing material canbe substantially the same as the size distribution of the comminutedcarbon.

In optional step 124, most, or all, of the coarsely sized carbon-basedmaterial is removed from (e.g., screened out of) the pre-treated slurry120 to form a carbon-based material-depleted slurry 136. As noted,removing the coarsely sized carbon-based material by screening in thecarbon-based material screen unit (not shown) is particularly effectivewhere there is a considerable size difference between the coarsely sizedcarbon-based material and other solid phases in the pre-treated slurry120. Screening can typically remove 95% or more of the coarsely sizedcarbon from the pre-treated slurry. The screened coarsely sizedcarbon-based material 128 may be directly recycled back to thepre-treatment step 116 and introduced into the mixer unit, typicallywithout requiring further washing or processing. Acid or basic washingcan be performed if required or desired.

The carbon-based material depleted slurry 136 or pre-treated slurry 120,as the case may be, is advanced to precious metal recovery step 140 inwhich the slurry 136 or 120 is contacted with a thiosulfate lixiviant toleach or dissolve most of the precious metal from the preciousmetal-bearing material. Dissolved precious metals can be recovered byknown techniques, such as resin-in-leach, cementation, precipitation,electrolysis, carbon adsorption, and the like, to form a precious metalproduct 144.

The pulp density of solids (including the precious metal-bearingmaterial and carbon-based material) in the mixer unit may be designed toachieve the required solid pulp density for thiosulfate leaching with orwithout the removal of the carbon-based material 128. The carbon-basedmaterial-depleted slurry 136 can then be fed directly to thiosulfateleaching, without any filtration or water addition being necessary.

EXPERIMENTAL

The following examples are provided to illustrate certain aspects,embodiments, and configurations of the disclosure and are not to beconstrued as limitations on the disclosure, as set forth in the appendedclaims. All parts and percentages are by weight unless otherwisespecified.

Example 1 Baseline Gold Recovery with Thiosulfate and Cyanide Leaching

Three different gold-containing oxide ore samples (P₈₀ of 80 μm), wereleached with thiosulfate. The leaching was conducted at pH 8, adjustedwith calcium hydroxide, for 24 hours at 50° C. using 0.1M calciumthiosulfate, 50 ppm Cu, 0.5-1 L/min air and 20 mL/L resin. Nopre-treatment was carried out on the samples.

A second set of the samples were leached using the cyanidecarbon-in-leach pretreatment process. Table 2 summarizes the compositionof the ores and the gold recovery results by thiosulfate leaching oneach sample. The gold recovery of samples A and B is very low and goldrecovery from sample C is approximately 71%.

As shown in Table 1, the ores are fairly similar in nature, and besides,oxygen and silicon contain other compounds:

TABLE 1 Element Units Sample A Sample B Sample C Gold g/t 17.86 4.757.45 Calcium Wt. % 0.279 0.922 8.413 Magnesium Wt. % 0.039 0.043 1.559Iron Wt. % 1.719 0.842 0.965 Total Oxides Wt. % >95 >95 >95 Total CarbonWt. % <2 <2 <2

Table 2 shows the gold recovery by thiosulfate leaching and cyanidation.

TABLE 2 Gold Leaching Sample A Sample B Sample C Method % Recovery %Recovery % Recovery Thiosulfate 22.7 26.4 70.7 Leaching Cyanide 92.273.4 69.3 Leaching

As is demonstrated above, gold recovery by thiosulfate is in some casessignificantly lower than that achieved by cyanidation.

Example 2 The Effect of Carbon Pretreatment on Gold Recovery byThiosulfate Leaching

The same three oxide ore samples from Example 1 were pre-treated withactivated carbon in oxygenated water at atmospheric temperature andpressure for 24 hrs. The weight ratio of ore to activated carbon was 2:1in all three of the tests. Overall solid pulp density (inclusive of oreand activated carbon) of the slurry in the pre-treatment process wasabout 45%, which resulted, after carbon separation, in solid pulpdensity of 35% in ore-water slurry. The required oxygen gas was suppliedby sparging the slurry via industrially-pure oxygen gas with thesparging rate of 0.5 L O₂/L slurry/min. The oxidation-reductionpotential (“ORP”) of the slurry during pre-treatment was greater thanabout 100 mV and less than about 500 mV Ag/AgCl. As will be appreciated,the ORP employed depends on the type of ore and the slurry makeup.

The activated carbon was screened out from the pre-treated slurry, andthe slurries leached with thiosulfate as described in Example 1. Thegold recovery results of the leaching process are presented below inTable 3:

TABLE 3 Gold Leaching Sample A Sample B Sample C Method % Recovery %Recovery % Recovery Thiosulfate 22.7 26.4 70.7 Leaching Carbon 86.2 71.180.2 Pretreatment and Thiosulfate Leaching Cyanide Leaching 92.2 73.469.3

All three samples show a significant increase in gold recovery bythiosulfate leaching, after use of the carbon pretreatment process.Sample A does not achieve the recovery observed with cyanide leaching.Sample B shows a recovery similar to cyanide leaching. Sample C shows abetter recovery than with cyanide leaching

Example 3 Pretreatment with Oxygen Only

The same tests of Example 2 were repeated on the same ores, however, nocarbon was added to the pre-treatment process (i.e., ore was mixed inoxygenated water). The final gold recovery from the samples was verysimilar to those of Example 1. In other words, pre-treatment without thecarbon-based material has no beneficial effect on gold recovery bythiosulfate leaching.

Example 4 Pretreatment Duration

Sample B of Example 2 was pre-treated and leached with the identicalprocesses to those of Example 2, except the pre-treatment was conductedfor 6 hrs, instead of 24 hrs. Decreasing the pre-treatment duration from24 hours to 6 hrs decreased the gold recovery from 71.1% to 60.7%.

Example 5 Affect of Oxygen Concentration

Various oxygen containing gases, such as pure oxygen gas, air oroxygen-enriched air, may be used for oxygenating. Ore sample A waspretreated in the same manner described in Example 2 with the exceptionthat oxygen was supplied as (i) pure oxygen gas, and (ii) air. Followingpretreatment, thiosulfate leaching was performed as described inExample 1. The gold recovery was 86.2% when the pretreatment wasperformed with oxygen and 81.4% when it performed with air.

Example 6 Effect of Pretreatment Temperature

Two samples of gold-bearing oxide ore (>95% oxides, <1% Fe, <2% carbon,and 18.5 ppm Au) were pre-treated with activated carbon in oxygenatedwater at atmospheric pressure for 24 hrs. The weight ratio of ore toactivated carbon was 2:1 in all three of the tests. Overall solid pulpdensity (inclusive of ore and activated carbon) of the pre-treatmentprocess was about 45%, which resulted in solid pulp density of 35% inore-water slurry, after carbon separation. One sample was pre-treated at25° C., and the other at 50° C. The obtained gold recoveries afterpre-treatment at 25 and 50° C. were 80.8% and 82.2%, respectively. Goldrecovery from the ore, without pre-treatment (i.e., direct thiosulfateleaching of the ore), was only 44.7%.

Example 6 Effect of Carbon Particulate Size

A series of test were conducted on the same ore sample. A baseline testusing standard carbon in leach (CIL) cyanidation techniques yielded agold extraction of 90.1%. Leaching of the same ore using thiosulfatesolution (0.1M calcium thiosulfate, 50 ppm Cu, 0.5-1 L/min air and 20cc/L resin, pH adjusted with calcium hydroxide) yielded a 57.4% goldrecovery after twenty four hours leaching.

In tests three through six, the sample was pretreated in 1 litre ofwater for 6 hours in the presence of coarse activated carbon and/orfinely ground activated carbon. The coarse activated carbon wasseparated from the ore prior to thiosulfate leaching, while the finelyground carbon remained with the ore during leaching. All carbon additionregimens increase the thiosulfate gold extraction above the baseline of57.4%. The greatest improvement of gold extraction occurs when the oreis pretreated with coarse activated carbon at an ore to carbon ratio of2:1 (82.9%). High gold extraction (81.8%) also occurs when the sample ispretreated with finely ground carbon at an ore to carbon ratio of 20:1.This indicates that when finely ground carbon is added, a smaller amountof carbon is required to improve the gold recovery. Fine carbon can beadded separately or inter-ground with the ore.

TABLE 4 % Gold Test # Test Description Extraction 1 CIL cyanide leaching90.1 2 Thiosulfate leaching 57.4 3 Pretreatment of 150 g ore with 75 g82.9 coarse activated carbon, followed by thiosulfate leaching 4Pretreatment of 150 g ore with 1.5 g 75.6 ground activated carbon,followed by thiosulfate leaching 5 Pretreatment of 150 g ore with 7.5 g81.8 ground activated carbon, followed by thiosulfate leaching 6Pretreatment of 150 g ore with 15 g 79.0 ground activated carbon,followed by thiosulfate leaching

A number of variations and modifications of the disclosure can be used.It would be possible to provide for some features of the disclosurewithout providing others.

For example, while coarsely sized carbon is preferred to avoid the needto continuously add carbon into the slurry and to allow carbon recyclein a continuous mode of operation, finely sized carbon may be used. Whenusing fine carbon, the carbon can not only be introduced in thepretreatment stage but also added into the grinding stage to grind theprecious metal-bearing feed material and carbon together to form acombined precious metal-containing and carbon-containing feed to thepretreatment stage. Using fine carbon in this way can reduce the amountof carbon consumed to less than 1 part carbon and 2 parts preciousmetal-containing feed material. The oxidant can be added during grindingor thereafter to effect pretreatment.

The present disclosure, in various aspects, embodiments, andconfigurations, includes components, methods, processes, systems and/orapparatus substantially as depicted and described herein, includingvarious aspects, embodiments, configurations, subcombinations, andsubsets thereof. Those of skill in the art will understand how to makeand use the various aspects, aspects, embodiments, and configurations,after understanding the present disclosure. The present disclosure, invarious aspects, embodiments, and configurations, includes providingdevices and processes in the absence of items not depicted and/ordescribed herein or in various aspects, embodiments, and configurationshereof, including in the absence of such items as may have been used inprevious devices or processes, e.g., for improving performance,achieving ease and\or reducing cost of implementation.

The foregoing discussion of the disclosure has been presented forpurposes of illustration and description. The foregoing is not intendedto limit the disclosure to the form or forms disclosed herein. In theforegoing Detailed Description for example, various features of thedisclosure are grouped together in one or more, aspects, embodiments,and configurations for the purpose of streamlining the disclosure. Thefeatures of the aspects, embodiments, and configurations of thedisclosure may be combined in alternate aspects, embodiments, andconfigurations other than those discussed above. This method ofdisclosure is not to be interpreted as reflecting an intention that theclaimed disclosure requires more features than are expressly recited ineach claim. Rather, as the following claims reflect, inventive aspectslie in less than all features of a single foregoing disclosed aspects,embodiments, and configurations. Thus, the following claims are herebyincorporated into this Detailed Description, with each claim standing onits own as a separate preferred embodiment of the disclosure.

Moreover, though the description of the disclosure has includeddescription of one or more aspects, embodiments, or configurations andcertain variations and modifications, other variations, combinations,and modifications are within the scope of the disclosure, e.g., as maybe within the skill and knowledge of those in the art, afterunderstanding the present disclosure. It is intended to obtain rightswhich include alternative aspects, embodiments, and configurations tothe extent permitted, including alternate, interchangeable and/orequivalent structures, functions, ranges or steps to those claimed,whether or not such alternate, interchangeable and/or equivalentstructures, functions, ranges or steps are disclosed herein, and withoutintending to publicly dedicate any patentable subject matter.

What is claimed is:
 1. A process, comprising: a) providing a particulatecarbon comprising one or more of activated carbon, activated charcoal,coke, hard carbon derived from at least one of coconut shells andelemental carbon, a calcined resin, and mixtures thereof; b) providing aprecious metal-containing material having a first precious metalthiosulfate leaching value in the absence of prior contact with theparticulate carbon; c) contacting the precious metal-containing materialwith the particulate carbon and an oxidant to form a pre-treated slurrycomprising a pre-treated precious metal-containing material and theparticulate carbon; and d) contacting the pre-treated slurry withthiosulfate to leach a precious metal from the pre-treated preciousmetal-containing material to form a precious metal-pregnant thiosulfateleach solution and a precious metal-barren material, wherein thepre-treated precious metal-containing material has a second preciousmetal thiosulfate leaching value after the contacting step c) that ismore than the first precious metal thiosulfate leaching value.
 2. Theprocess of claim 1, wherein, in step c), the precious metal-containingmaterial is free of contact with thiosulfate.
 3. The process of claim 1,wherein the precious metal-containing material contains more oxides thansulfides and wherein the precious metal-containing material isrefractory to thiosulfate leaching.
 4. The process of claim 1, whereinthe precious metal-containing material is an oxide ore that is notcyanide refractory.
 5. The process of claim 1, wherein the oxidant ismolecular oxygen, wherein the precious metal-containing material beforestep c) has an initial pH of at least pH 3, wherein the pre-treatedslurry has a pH of from about pH 7 to about pH 10, wherein anoxidation-reduction potential during step c) ranges from about 100 toabout 750 mV (Ag/AgCl electrode), and wherein a rate of contact ofmolecular oxygen with the slurry during step c) is at least 0.10 L O₂/Lslurry/min.
 6. The process of claim 1, wherein a weight ratio of theprecious metal-bearing material to the particulate carbon ranges fromabout 50:1 to about 1:0.01 and wherein after recovering the preciousmetal from a precious metal-pregnant leach solution, the particulatecarbon is sent to tailings along with the precious metal-barrenmaterial.
 7. The process of claim 1, wherein the pre-treated slurry iscontacted with thiosulfate in the absence of a pH adjustment and/or aslurry density adjustment.
 8. The process of claim 1 further comprisingremoving the particulate carbon from the pre-treated slurry, wherein theprecious metal-containing material has an average preciousmetal-containing material particle size, wherein the particulate carbonhas an average carbon particle size, and wherein the average carbonparticle size is more than the average precious metal-containingmaterial particle size and wherein the particulate carbon is removedfrom the pre-treated slurry by screening.
 9. A process, comprising: a)contacting a gold-containing material with particulate carbon and anoxidant, the particulate carbon comprising one or more of activatedcarbon, activated charcoal, coke, hard carbon derived from at least oneof coconut shells and elemental carbon, a calcined resin, and mixturesthereof, to form a pre-treated slurry comprising a pretreatedgold-containing material and the particulate carbon; b) leaching thepre-treated slurry with thiosulfate to dissolve gold in the pre-treatedslurry and form a gold pregnant thiosulfate leach solution and agold-barren material; and c) recovering dissolved gold from the goldpregnant leach solution by one or more of resin-in-leach, cementation,precipitation, and electrolysis.
 10. The process of claim 9 furthercomprising removing the particulate carbon from the pre-treated slurry,wherein the precious metal-containing material has an average preciousmetal-containing material particle size, wherein the particulate carbonhas an average carbon particle size, and wherein the average carbonparticle size is more than the average precious metal-containingmaterial particle size and wherein the particulate carbon is removedfrom the pre-treated slurry by screening.
 11. The process of claim 9,wherein, in step a), the gold-containing material is free of contactwith thiosulfate.
 12. The process of claim 9, wherein thegold-containing material contains more oxides than sulfides and whereina gold recovery by thiosulfate leaching of the gold-containing materialin the absence of prior contact with the carbon is less than a goldrecovery by thiosulfate leaching of the gold-containing material afterprior contact with the carbon.
 13. The process of claim 9, wherein thegold-containing material is an oxide ore that is not cyanide refractory,wherein the carbon-depleted slurry has a greater gold thiosulfateleaching value than at least part of the gold-containing material, andwherein at least part of the gold-containing material is refractory torecovery of the gold by thiosulfate leaching.
 14. The process of claim9, wherein the oxidant is molecular oxygen, wherein the gold-containingmaterial before step a) has a pH at least pH 3, wherein, during step a),the slurry has a pH of from about pH 7 to about pH 10, wherein, duringstep a), the slurry has an oxidation-reduction potential ranging fromabout 100 to about 750 mV (Ag/AgCl electrode), and wherein a rate ofcontact of molecular oxygen with the slurry during step a) is at least0.10 L O₂/L slurry/min.
 15. The process of claim 9, wherein a weightratio of the gold-bearing material to carbon ranges from about 50:1 toabout 1:0.01 and wherein after recovering dissolved gold from the goldpregnant leach solution, the particulate carbon is sent to tailingsalong with the gold-barren material.
 16. The process of claim 9, whereinthe pre-treated slurry is contacted with thiosulfate in the absence of apH adjustment and/or a slurry density adjustment.
 17. The process ofclaim 16, wherein a pH of the pre-treated slurry is adjusted by no morethan pH 0.1 and a slurry density is adjusted by no more than 5%.
 18. Aprocess, comprising: a) contacting a gold-containing material withcarbon and an oxidant to form a pre-treated slurry, the carboncomprising one or more of activated carbon, activated charcoal, coke,hard carbon derived from at least one of coconut shells and elementalcarbon, a calcined resin, and mixtures thereof; and b) contacting thepre-treated slurry with thiosulfate to leach gold from the pre-treatedslurry, wherein: in step a), the pre-treated slurry comprises less than0.005 molar thiosulfate; the gold-containing material contains moreoxides than sulfides; the gold-containing material comprises an oxideore that is not cyanide refractory; a gold recovery by thiosulfateleaching of the pre-treated slurry is greater than a gold recovery bythiosulfate leaching of the gold-containing material in the absence ofprior contact with the carbon; the oxidant is molecular oxygen; thegold-containing material before step a) has a pH at least pH 3; duringstep a), the gold-containing material has a pH of from about pH 7 toabout pH 10; during step a), the gold-containing material has anoxidation-reduction potential ranging from about 100 to about 750 mV(Ag/AgCl electrode); a rate of contact of molecular oxygen with theslurry during step a) is at least 0.10 L O₂/L slurry/min; and a weightratio of the gold-bearing material to carbon ranges from about 1:3 toabout 1:0.01.
 19. The process of claim 18, wherein the pre-treatedslurry is contacted with the thiosulfate in the absence of a pHadjustment and/or a slurry density adjustment and wherein at least partof the gold-containing material is refractory to recovery of gold bythiosulfate leaching.
 20. The process of claim 18 further comprisingremoving at least about 95% of the carbon from the pre-treated slurryprior to step b), wherein the carbon is removed from the pre-treatedslurry by screening.